28 December 2008

Who killed the coral reefs?

"The corals are gone and the big fish are gone. That's happened in my lifetime." says marine scientist John Bruno.

The alarming rate at which corals have died off in recent decades concerns scientists and has stirred debate about what is causing the loss. They're trying to tease out how much of the loss is part of natural disruptions that cause coral to die off periodically and how much is caused by man-made activities such as fishing and pollution.

"It's a wonderful murder mystery for ecologists," says Bruno. "It's not obvious what the cause is. There are lots of potential culprits."Death of corals is oceanographer's murder mysteryUNC scientist says there are many suspected culpritsWade Rawlins, The News & Observer 28 Dec 08;

Marine scientist John Bruno became interested in coral reefs as a boy snorkeling in the turquoise waters off the Florida Keys above reefs of golden corals the size of football fields.

"It just went on for acres and acres," recalls Bruno, 43, an associate professor at UNC-Chapel Hill. "They were just full of fish. We'd see hammerhead sharks on the reef and big critters. That is all gone.

"The corals are gone and the big fish are gone," he says. "That's happened in my lifetime."

Healthy coral reefs are as vibrantly alive as tropical rain forests, providing habitat and cover to tens of thousands of fish and other marine animals. They're rarer than rain forests, covering far less than 1 percent of the Earth, and are disappearing much faster. As corals die, the abundance of reef fish declines because of a lack of habitat for baby fish to start life.

The alarming rate at which corals have died off in recent decades concerns scientists and has stirred debate about what is causing the loss. They're trying to tease out how much of the loss is part of natural disruptions that cause coral to die off periodically and how much is caused by man-made activities such as fishing and pollution.

"It's a wonderful murder mystery for ecologists," says Bruno, who has been the studying the effects of disease and warming sea water on coral reefs. "It's not obvious what the cause is. There are lots of potential culprits."

Among the suspects are pollution, destructive fishing practices, predators that feed on corals, disease and warmer ocean waters.

In the ocean, reef-building corals, which are marine polyps, a class of animals, typically exist in colonies of many identical individuals. They fill the role of trees in a forest, Bruno says. The skeletons of corals create the hardened framework of a reef and, over time, build up and provide habitat for thousands of other animals and plants. Corals require warm, clear water and are sensitive to temperatures.

A warming of the ocean by just a degree or two for a few weeks in summer can disrupt the life cycle of corals, Bruno says. Reef-building corals contain tiny plant-like algae that live within their tissue in a mutually beneficial relationship. The algae provide the coral with food and oxygen, as well as the vibrant colors for which corals are known. In return, the organisms receive shelter and nutrients.

But when water temperatures remain elevated, the algae are expelled. Without the essential algae, the coral loses its color -- a phenomenon known as "bleaching." More important, it loses its source of nutrients, which often leads to the coral's death.

The largest coral bleaching that scientists have observed in the Caribbean occurred in 2005.

"The mass bleaching we're seeing is caused by warmer temperatures," Bruno says. "There is no question about that."

The bleaching followed an outbreak of a condition called white band disease, which caused widespread loss of elkhorn and staghorn corals in the Caribbean during the 1980s -- the beautiful golden corals that had transfixed Bruno as a youth.

"It's kind of like losing all the pine trees in Carolina and Georgia," Bruno says. "It just changed the ecosystem. That is pretty much what happened under water. We just didn't see it happen."

In 2006, elkhorn and staghorn corals were put on the endangered species list under the category of threatened, due to their widespread decline.

Researchers are trying to understand whether higher ocean temperatures are contributing to the outbreak and spread of disease.

Coral disease study

Bruno and his fellow researchers found a strong correlation between higher temperatures and the frequency of coral disease on Australia's Great Barrier Reef in a study published last year, though he says the disease defied easy predictability. They noted that major outbreaks occurred only where coral cover was abundant after especially warm years.

The results suggest that climate change could be increasing the severity of disease in the ocean and that predicted increases in temperatures in tropical oceans can make corals more vulnerable to disease in the future, Bruno says.

Bruno and colleagues published a large-scale analysis of the extent of coral loss in the central and western Pacific Ocean by compiling 6,000 surveys of more than 2,600 coral reefs done over 36 years. The region contains about three-quarters of the world's coral reefs.

The researchers found that since the late 1960s, nearly 600 square miles of reef have disappeared per year. That is twice the rate of the loss of rain forests.

Bruno said coral reefs provide economic benefits through fisheries and tourism and perform valuable services such as buffering coastal islands from storms. When corals die, these benefits quickly disappear, he said.

But Bruno doesn't think coral reefs will disappear. They show some ability to recover.

"I am cautiously optimistic, more so than many of my colleagues," Bruno says. "I think we are learning how to manage them better."

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